1. Introduction
The digital storage oscilloscope (DSO) is a widely used in time-domain measurement, and the waveform capture rate (WCR) is an important index to evaluate the performance of DSO's data acquisition system. The WCR can be defined as “the number of waveforms that can be captured in a unit time (wfms/s)”. It indicates the size of amount of information that the acquisition system can capture and display within the unit time. The higher WCR indicates the stronger capacity of the oscilloscope for capturing transient signal.
The WCR of DSO has been improved dramatically over the last ten years. For example, the WCRs of Tektronix Digital Phosphor Series Oscilloscopes range from 3600 wfms/s to 300,000 wfms/s, and the Infiniium 90000A Series DSOs from Agilent have achieved 400,000 wfms/s. Since the WCR is a critical indicator for DSO, all international instrument suppliers highlight the WCR as one of selling points.
We have put forward a method for measuring the waveform capture rate of DSO, which has been granted a patent on Jun. 1, 2011 with No. CN101281224B. The method is also called as “Double Pulses Measurement”, and fills the gap of measuring the WCR of DSO. However, with this method, only the transient WCR of DSO can be measured, and the results just reflect the WCR at measuring moment. The inherent deficiency of this method is that the measured WCR cannot be used to evaluate capturing capacity during a certain time of interval. Due to the different techniques used by DSO manufacturers in the system structure and waveform display process, there may exist large bias when this method is adopted to measure the WCR of parallel DSO.
2. Review of Waveform Capture Rate
FIG. 1 is a diagram of dead time of data acquisition system.
The relation between actual dead time and acquire time, and the relation between effective dead time and display window are shown in FIG. 1.
The operation mode of DSO alternates between acquisition and processing of waveform data. After acquiring the waveform data, the microprocessor unit (MPU) of DSO will be involved in the processing of the acquired waveform data. DSO will not acquire the waveform data, while processing. Therefore, there exists a time gap between two waveform data acquisitions, and this “time gap” is called actual dead time. Obviously, the actual dead time is the time interval from the end of previous waveform data acquisition to the start of current waveform data acquisition. The effective dead time include actual dead time and the part of acquire time which is out of the display window. Part of waveform data in acquire time can be acquired, but can not be displayed. If the fault signal is located in the part of waveform, for instance, the second circle as shown in FIG. 1, the waveform data of the fault signal can be acquired, but can not been displayed. Therefore, the effective dead time is more important than the actual dead time in analyzing the performance of DSO, the dead time hereinafter referred to is effective dead time.
DSO starts to acquire the waveform data after the end of previous acquisition period, and does not monitor and capture the signal, thus leading to the loss of fault signal and a deceptive waveform display.
The occurrence of failure in circuit system generally does not follow any law, and it is very difficult for us to select an appropriate trigger condition to capture the fault signal. Therefore, the high WCR of DSO is very important to find the failure of circuit system and enhance the efficiency of measurement.
3. Deficiency of the Method for Measuring the WCR of DSO in Prior Art
3.1 Double Pulses Measurement
For the first time, the method for measuring the WCR of DSO in prior art with double pulses solves the measurement problem of WCR through the external characteristic.
The principle of double pulses measurement is that, acquisition process is controlled by the trigger signal, and the time interval between two consecutive effective triggers would be the dead time.
As shown in FIG. 2, the measuring signal consists of consecutive pulse W1 and pulse W2, the width of pulse W1 is less than that of pulse W2. The rising edges of two pulses are trigger position t1 and t2, and T0 is an adjustable time interval between trigger position tl and t2. When the time interval T0 is less than the dead time of DSO, the acquisition system of DSO can only capture and display the pulse W1; when the time interval T0 is greater than the dead time of DSO, the acquisition system of DSO can capture and display both pulse W1 and pulse W2; when the time interval T0 is adjusted exactly to critical time point that both pulse W1 and W2 can just be viewed, then the time corresponding to time interval T0 is the dead time of DSO.
In double pulses measurement, if the time interval T0 is small enough, there is no time for DSO to proceed with the next acquisition triggered by pulse W2 after pulse W1 is acquired due to the existence of the dead time, thus leading to the loss of pulse W2. By increasing the time interval T0, the measurement of WCR shall be completed when DSO can just capture the two pulse signals.
For convenience to observe, the two pulses have different widths. The observer can clearly determine the critical point for the occurrence or no occurrence of pulse W2. Since this critical point exactly demonstrates the shortest time of an acquisition, it ranges from the rising edge of the first pulse to that of the second pulse. This time interval is represented as T0min.
The time interval T0min is the shortest time between two effective triggers, and also the shortest time needed in an acquisition and processing of DSO. Its reciprocal is the maximal WCR of DSO, and can be written as:WCRmax−1/T0min   (1).3.2. Architecture of DSO
The conventional DSO is based on the serial structure, as shown in FIG. 3, 4. The waveform data acquired is sent to microprocessor, then processed, and finally displayed. The operation of serial DSO can be described as “acquiring a piece of waveform data, then processing slowly and displaying, and repeating the steps”. The DSO would not monitor the signal under test, while “processing slowly”. The period of “processing slowly” is “dead time”. Generally, DSO based on serial structure can capture only 1% of waveforms and 99% of waveforms are lost within the “dead time”, and that make the measurement inefficient. In other word, the rate to capture the signal under test is very low.
With the development of the DSO, the WCR has been given more and more considerations. Reducing the dead time as short as possible by improving the structure of the DSO's acquisition system and changing the acquisition and display mode is the key to enhance the WCR of DSO. In the late 1990s, Tektronix first developed a DSO with parallel structure, called Digital Phosphor Oscilloscope. Since that, the WCR of DSO has been improved dramatically.
As shown in FIG. 5, 6, DSO with parallel structure consists of three main parts: signal conditioning and triggering module, data acquisition and waveform parallel processor module, and microprocessor and display module.
Analog signal, i.e. signal under test is fed into ADC after conditioning, and is sampled under the control of trigger circuit and time base circuit. The waveform data sampled are delivered to the acquire storage. After a waveform data acquisition is completed, the waveform parallel coprocessor will map the waveform data in acquire storage into a waveform database, which corresponds to the dot-matrix. When the mapping is over, a new round of waveform acquisition and mapping will start. Meanwhile, the microprocessor will execute the calculation of waveform, menu management and man machine interface management. When the refresh time of LCD arrives, the display refresh controller will be started up, and combine the dot-matrix data in waveform database with that in interface database, then import the combined dot-matrix data into the display storage and refresh the display.
In parallel DSO, the acquisition and processing of waveform data and the operation of MPU are parallel, the MPU can extricate it from processing of waveform data and displaying. And the parallel architecture employed in DSO can reduce the dead time, and increase probability of capturing transient signal.
The way of capturing the signal under test by parallel DSO is that DSO acquires and maps the waveform data repeatedly, when refresh time of LCD arrives, the DSO stops the acquiring and mapping, and the waveform parallel coprocessor export the mapped data of plurality of waveforms to display storage. So we can see that the dead time of parallel DSO consists of two parts: the time introduced by waveform mapping and the time introduced by exporting the mapped waveform data at refresh time arriving.
3.3. Deficiency of Double Pulses Measurement
From the analysis of architecture of DSO, we can see that the double pulses measurement has significant limitations. Only the capture of two consecutive waveforms is scaled, therefore, the WCR obtained by double pulses measurement is a transient waveform capture rate of DSO. For serial DSO, since the time intervals between two acquisitions are usually symmetrical, the WCR obtained by double pulses measurement basically reflects the WCR in a unit time, the error is small. However, for parallel DSO, due to the particularity of its structure and mapping method, there are two different phases, each phase has a different dead time, and the WCRs in a refreshing period are asymmetrical. In such case, if the double pulses measurement is employed to evaluate the WCR, the results would be wrong, and much higher than the real value.
The parallel DSO is becoming a mainstream product in DSOs, a method for measuring the waveform capture rate of parallel digital storage oscilloscope is needed.